Canon: AI AF modes - DPChallenge Forums

06/15/2004 05:46:52 PM · #1

There was a bit of discussion on this topic a week or so ago, but

I was shooting a sporting event at the weekend, with runners, running towards me. I'm shooting with a D60, 70-200 F4l (mostly at the 200mm end) in Av mode, set at F4. Bright sunny day, with most of the shutter speeds up around 1/500s or so.

The lens is set to auto focus, the body set to AI servo (I.e., not 'one shot mode) and I'm tracking the people as they essentially run towards me.

The people largely cover all of the 3 focal points (shooting in portrait mode). As I see someone, I would half press the shutter, and the lens would supposedly start tracking. Roughly 50% of the shots have the subject softly focused (i.e., out of focus) I don't believe it is camera shake, or that the they are running - the other 50% are sharp, crisp and clear.

So is there something I'm missing on how the AI focus modes work ? Is there a specific technique to getting a good lock ? Any suggestions would be great.

Edit: I was shooting in multi-shot drive mode, so I can shoot without the camera locking focus - not sure if that is a factor too.

Message edited by author 2004-06-15 17:47:35.

06/15/2004 05:48:55 PM · #2

Quick question first - what ISO?

06/15/2004 05:51:31 PM · #3

ISO 100, or 200 at the most.

RonB

Canon EOS-300D Rebel

06/15/2004 05:55:57 PM · #4

Is it possible that you were actually engaging the shutter while the AF was in motion? F4 doesn't have much DOF at 200mm, so would be pretty sensitive to the smallest movement.

Ron

06/15/2004 05:57:55 PM · #5

I would have been very tempted to shoot at 400, possibly with -1/3Ev, depending on just how bright it is.

Here's a heavily cropped (about 1.5px:1) shot I took the other day at 400:

and at 1:1

Edit: Those are straight out of the camera.

Message edited by author 2004-06-15 17:59:08.

06/15/2004 05:59:27 PM · #6

Originally posted by PaulMdx:

I would have been very tempted to shoot at 400, possibly with -1/3Ev, depending on just how bright it is.

For many of the shots, the shutter speed at ISO 100 was at 1/1000th (with a -0.5EV dialed in, so I don't believe shutter speed is the problem or subject motion. Easily 50% of the shots are sharply focused, crisp and good enough to print large. The rest are just out of focus.

06/15/2004 06:00:11 PM · #7

Originally posted by RonB:

Is it possible that you were actually engaging the shutter while the AF was in motion? F4 doesn't have much DOF at 200mm, so would be pretty sensitive to the smallest movement.

Ron

Absolutely - the AI servo modes are continually focusing. As I understand it, that's the point - it is supposed to track a moving subject (like someone running towards you)

06/15/2004 06:04:14 PM · #8

Originally posted by Gordon:

It's not the shutter I'm thinking about - it's the DOF. Sorry - I meant to go on to say I'd then increase the f-stop to compensate. So if you were getting 1/1000 at ISO 100, I'd probably be aiming for 1/1000 on ISO 400 and a higher f-stop. I tend to do this so I'm a little more likely to keep the subject within the DOF.

I must admit, mostly because of lack of specifically selecting Ai-Servo on an un-hacked 300D, I don't really use Ai-Servo. Rather, I keep tapping the shutter release to keep it beeping and in focus, then jam my finger down to (re)focus/shoot. If I know the subject will stay in focus (which it won't in your case) I may keep my finger down for multiple shots. Otherwise I'd release my finger completely and jam it down again. It sounds fairly stupid, but you're relying on the fast response of the camera, and normally I tend to find it works ok, even with my old 75-300.

Message edited by author 2004-06-15 18:09:03.

tomlewis1980

Canon EOS-10D

Canon EF 50mm f/1.8 II

06/15/2004 06:04:30 PM · #9

With the 10D I always find the AI servo mode works best with just one AF point selected, I guess the same goes for the D60?

06/15/2004 06:08:42 PM · #10

Originally posted by tomlewis1980:

With the 10D I always find the AI servo mode works best with just one AF point selected, I guess the same goes for the D60?

Could well be - I've had problems in the past using one focus point, so was trying all 3 in this case, and shooting portrait (which, with the 3 point AF on the D60 makes it basically a big center)

next time I'll switch back down to one point (which is what I normallly use on one shot modes)

06/15/2004 06:10:38 PM · #11

Originally posted by PaulMdx:

Rather, I keep tapping the shutter release to keep it beeping and in focus, then jam my finger down to (re)focus/shoot. If I know the subject will stay in focus (which it won't in your case)

Stopping down a bit would certainly increase the DoF a bit, true. I like the blurry backgrounds though when it actually does get in focus.

And one shot mode, like you mention is a certain way to get an out of focus subject in this case, by the time the camera has focused, I recognise this and press the shutter, the subject will be too close to be in focus.

I had better results last week with all manual tracking/focus.

06/15/2004 06:16:40 PM · #12

Originally posted by Gordon:

Stopping down a bit would certainly increase the DoF a bit, true. I like the blurry backgrounds though when it actually does get in focus.

You're just too picky. ;-) At 200mm surely you get a reasonable brokeh?

Originally posted by Gordon:

And one shot mode, like you mention is a certain way to get an out of focus subject in this case, by the time the camera has focused, I recognise this and press the shutter, the subject will be too close to be in focus.

If the camera is near in focus to the subject refocusing should be quick, and if you press the button fully it will take a picture within a reasonable timeframe, I tend to find.

06/15/2004 06:21:09 PM · #13

Originally posted by PaulMdx:

Originally posted by Gordon:
Stopping down a bit would certainly increase the DoF a bit, true. I like the blurry backgrounds though when it actually does get in focus.

You're just too picky. ;-) At 200mm surely you get a reasonable brokeh?

If the camera is near in focus to the subject refocusing should be quick, and if you press the button fully it will take a picture within a reasonable timeframe, I tend to find.

I could be less picky, but people manage to do this with longer lenses at wider apertures - I'm trying to understand how :) Also, while this was in bright sunlight and I'd have the luxury of shooting f5.6 and still having a fast enough shutter, I'd rather be able to use f4 with good technique to get more light.

On the second point, in one shot mode the camera does focus quickly, the problem is that the subject is running consistantly towards the camera, so the focus point will instantly be behind them when it focuses, and by the time I've taken the shot it will be out of focus. I tried this quite a lot too to explore different approaches. They were all (every one) soft to one degree or another (and from people behind the subject, it was obvious that the focus was where they were when I focused it, not where they were when I took the shot)

Message edited by author 2004-06-15 18:22:07.

06/15/2004 06:23:39 PM · #14

Originally posted by Gordon:

I could be less picky, but people manage to do this with longer lenses at wider apertures - I'm trying to understand how :)

With what camera though?

Originally posted by Gordon:

On the second point, in one shot mode the camera does focus quickly, the problem is that the subject is running consistantly towards the camera, so the focus point will instantly be behind them when it focuses, and by the time I've taken the shot it will be out of focus. I tried this quite a lot too to explore different approaches. They were all (every one) soft to one degree or another (and from people behind the subject, it was obvious that the focus was where they were when I focused it, not where they were when I took the shot)

Fair enough - I can only go off my experience. I'm not saying it's the right/best way, just saying what has worked for me in the past.

06/15/2004 06:30:08 PM · #15

Originally posted by PaulMdx:

Originally posted by Gordon:
I could be less picky, but people manage to do this with longer lenses at wider apertures - I'm trying to understand how :)

With what camera though?

Where was that 1D MkII review again...

Canon EF 100-400mm f/4.5-5.6L IS

06/15/2004 06:33:05 PM · #16

Originally posted by Gordon:

Where was that 1D MkII review again...

LOL... I was reading just yesterday that even the focus of a 70-200/2.8L on a 10D is supposed to be nothing compared to that on a 1D mkII..

Where was that loan application...

ellamay

Canon EOS-1D Mark II

06/15/2004 06:39:34 PM · #17

this is probably going to sound really silly, but I hate AI servo mode, even with mark II, I have not found it a reliable way to track, on the rebel I found it better to just track subject myself, I am trying to learn it again on the mark ii cuz seems dumb to have such a fast camera and not use it. But I like my shots better when use center focusing and track it myself.
This however may be useful to you--
Cat=&Number=174243&page=&view=&sb=5&o=&fpart=all&vc=1>
it is one of the key canon guys talking about it--maybe you will have better luck than I did with getting this mode to work
p.s. if you find a way to have lots of success do let us know

Message edited by author 2004-06-15 18:41:21.

06/15/2004 08:12:24 PM · #18

Originally posted by ellamay:

When you say 'track it yourself' what do you mean ? That you are doing manual focusing ? But you mention using center focusing, so that confuses me. Thanks for the link - I'll check it out.

I'm talking about trying to focus on a subject that is moving directly towards the camera, at between 10 to 20 mph, typically (running or cycling mostly)

Message edited by author 2004-06-15 20:13:00.

Canon EF 100-400mm f/4.5-5.6L IS

06/15/2004 08:21:30 PM · #19

Of particular interest from the robgalbraith post was

//photonotes.org/other/ai-servo.html

ellamay

Canon EOS-1D Mark II

06/15/2004 08:40:52 PM · #20

I am on single shot mode, with center focus and I track myself,literally by moving the camera with the subject, is that clearer gordon?

Canon EF 70-200mm f/2.8L IS USM

06/15/2004 08:46:59 PM · #21

Originally posted by ellamay:

I am on single shot mode, with center focus and I track myself,literally by moving the camera with the subject, is that clearer gordon?

Yup - thanks!

EddyG

Canon EOS-1D Mark II

06/15/2004 10:22:15 PM · #22

As a point of reference, the Mark II seems to track fine in a situation similar to what you describe. When I first got it, I put together this QuickTime movie to demonstrate 8.5FPS to a friend of mine. It is a 22-shot sequence, shot in AI Servo mode hand-held with the 70-200/2.8L IS with the 1.4X TC attached, wide-open at Æ�/4.

One thing that changed with the Mark II is that it has focus-priority on the first shot of a multi-burst sequence, unlike previous Canon DSLR's. That means that the first shot of a burst sequence can be out of focus on other cameras, because of the shutter-priority nature. The Mark II has dual AF CPU's, and is designed to get accurate focus with single shots in AI Servo or the first shot in a burst. This way, you get the benefits of a release-priority system in terms of capturing the decisive moment, together with the predictive AF accuracy you need with moving subjects.

(I've noticed on some occasions when shooting bursts with the Mark II in low-light where there will be slight pauses in an otherwise very rhythmic shutter as the camera waits for focus to lock to try and avoid a blurry shot.)

Message edited by author 2004-06-15 22:28:35.